Metallic materials are used in elemental form, as alloys or as composites in various mechanical devices, chemical devices, etc. depending on their physical and chemical properties. When they are used as parts requiring corrosion resistance, the surfaces of such parts only need to have sufficient corrosion resistance. It has been the practice therefore to coat the surface of a metal substrate with a material having superior corrosion resistance.
For example, it is known that titanium exhibits excellent corrosion resistance by forming a passive oxide film on the surface thereof. Thus, titanium has recently gained acceptance as a material for various machines, appliances and instruments such as chemical devices. In particular, in electrolysis apparatuses for sea water, saline water, etc., pure titanium has been used widely as a material for an electrolytic cell or a substrate of an insoluble metallic electrode. As such, however, crevice corrosion, etc. still tends to occur with pure titanium. The corrosion resistance of pure titanium is still not sufficient when titanium is used as an electrode substrate in electrolysis of strongly acidic electrolytic solutions containing hydrochloric acid sulfuric acid, etc. Attempts have therefore been made to coat the surface of titanium with platinum-group metals, such as palladium, or their alloys, or anticorrosive metals such as tantalum or niobium and their alloys.
Various methods have been suggested to date for forming a coating of an anticorrosive metal on the surface of a metal substrate. For example, Japanese Patent Publication No. 415/68 and Japanese Patent Application (OPI) No. 19672/75 disclose a method for preventing crevice corrosion by bonding a titanium-palladium alloy material to a titanium substrate by welding, and the like. Bonding by welding, however, requires a high level of welding skill. It is difficult to apply this method to materials with a complex profile, and the strength of adhesion of such a material to the substrate is not entirely satisfactory.
On the other hand, various methods are known for depositing an anticorrosive material on the surface of a metal substrate by electroplating, chemical (electroless) plating, thermal decomposition, spraying, vacuum deposition, etc., to coat the surface with such a material, and heat-treating the coated substrate (see, for example, Japanese Patent Publication Nos. 12882/71, 2669/73 and 24136/73, and Japanese Patent Application (OPI) Nos. 25636/73, 40676/73, and 4736/78). According to these methods, the thickness of the coating can be made as thin as is required. However, formation of micropores in the coated layer cannot be avoided, and heat-treatment must be performed in a vacuum, etc., for a long period of time. Because of these difficulties, the prior art methods have been unable to provide products having a high degree of corrosion resistance and satisfactory adhesion of the coated layer to the substrate.